This invention relates to sample data filtering, and more particularly to a hybrid time multiplexing sample data filter using delta modulation.
The conventional delta modulator uses a 1-bit quantizer (comparator) which is clocked every .tau. seconds to produce a binary pulse sequence output, x(t), ##EQU1## WHERE ##EQU2## AND S(T) IS A RECTANGULAR PULSE OF WIDTH .tau.. The signal x is the demodulated version of x (t) and is obtained by passing the binary signal through a low pass filter which can be an integrator. The gain of the filter has a scaling factor that can be adjusted so that the occurrence of slope overload can be made sufficiently infrequent. The demodulated signal exhibits a quantizing noise which depends jointly on the sampling rate (1/.tau.) and the step size (.DELTA.) which is the amount of jump in the response of the filter to a single pulse, s(t).
The feature of primary significance is that demodulation is accomplished with a linear, time-invariant network which means that the linear operations of multiplication by a constant (scaling), summation, and delaying, to be performed on the demodulated signal, can equivalently be performed on the delta-encoded signal. For the nonrecursive case, these operations on x(t) can be performed by a device known as a binary transversal filter (BTF). This device consists of a shift register (clocked at rate 1/.tau.) to provide delayed versions of the signal, and a resistance weighting network to scale and sum the individual terms. The BTF has received considerable attention as a means for generating digital signalling waveforms. The shift register is tapped at every k.sup.th stage to provide the signals into the resistance weighting network. Negative scaling factors are accommodated by having a separate summing node for one of the inputs to a difference amplifier. The parameter k = T/.tau. is the ratio of sampling rate in .DELTA.M to sampling rate in PCM and it essentially characterizes the error in the .DELTA.M conversion process. Depending on accuracy requirements, values of k between 10 and 1000 might be needed.
The binary transversal filter performs only the delay operation digitally. The scaling and summing operations require careful selection of resistance values and one of the desirable features of a digital filter is missing. The digital filter allows arbitrary scaling coefficients to be programmed in so that one structure can serve many filtering functions.
The present invention incorporates the delta modulation scheme by means of digitally-controlled pulsewidth modulation. The demodulator response is essentially independent of the shape of the individual binary pulses since their bandwidth greatly exceeds the bandwidth of the demodulator. Hence the demodulator output can be scaled by changing the width, rather than the height, of the pulses in the .DELTA.M signal. Assuming that prescaling has been done so that .vertline.aj.vertline..ltoreq.; j = 0, 1, ..., N-1, the outputs of the pulsewidth modulators at each tap are combined in an analog fashion to provide the signal. ##EQU3## and y(t) = F.y(t) is the desired approximation to the nonrecursive version.